. Bulletin of the British Museum (Natural History), Geology. FROM ENGLAND 137 Diagnosis. A genus with crioceratitid coiling and strong, single, radial ribs. Tubercles, mainly ventro-lateral, are common in juveniles but disappear with growth. Description. The maximum diameter is at least 500 mm, the body chamber occupying \ to § of a whorl. Coiling is crioceratitid throughout ; adjacent whorls may be almost in contact or well separated. The whorl section is rounded to sub- quadrate, inflated to compressed, the dorsum varying from slightly impressed to slightly convex. Ornament consists of
. Bulletin of the British Museum (Natural History), Geology. FROM ENGLAND 137 Diagnosis. A genus with crioceratitid coiling and strong, single, radial ribs. Tubercles, mainly ventro-lateral, are common in juveniles but disappear with growth. Description. The maximum diameter is at least 500 mm, the body chamber occupying \ to § of a whorl. Coiling is crioceratitid throughout ; adjacent whorls may be almost in contact or well separated. The whorl section is rounded to sub- quadrate, inflated to compressed, the dorsum varying from slightly impressed to slightly convex. Ornament consists of strong, single, rectiradiate, prorsiradiate or rursiradiate, straight or curved ribs which extend across the venter without inter- ruption but are usually represented on the dorsum by forwardly curving, feeble rib- folds. The dorsum is also crossed by well-defined growth striae, which are less clearly visible on flanks and venter. On the early whorls small ventro-lateral tubercles are generally present on most ribs ; these disappear with growth. In one species (A. semicinctum), mid-lateral and sometimes dorso-lateral as well as ventro-lateral tubercles occur on occasional ribs in early growth Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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